A hydroformylation process comprises contacting under reaction conditions an olefinically-unsaturated compound with carbon monoxide and hydrogen in the presence of a catalyst to produce one or more aldehydes. For example, U.S. Pat. Nos. 4,148,830, 4,717,775, and 4,769,498 generally relate to hydroformylation processes using transition metal-organophosphorus ligand complex catalysts. Preferred transition metals include iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium and platinum. Rhodium is a more preferred transition metal. Organophosphines and organophosphites are preferred organophosphorus ligands. Aldehydes produced by hydroformylation processes are also referred to as oxo aldehydes, which have a wide range of utility, for example, as intermediates for hydrogenation to aliphatic alcohols, for amination to aliphatic amines, for oxidation to aliphatic acids, and for aldol condensation to plasticizers.
A transition metal-organophosphorus ligand complex catalyst produces in the hydroformylation process generally an isomeric mixture comprising a linear (normal or n-) aldehyde and one or more branched (iso- or i-) aldehydes. A ratio of the n-aldehyde to the sum of the iso-aldehydes, calculated by molar or by weight, is often described as N/I selectivity or N/I ratio. Since all isomeric aldehydes produced from a given olefinically-unsaturated compound have an identical molecular weight, the molar N/I ratio is identical to the weight N/I ratio. For the purposes of this invention, an N/I selectivity of a catalyst refers to the N/I ratio obtained from hydroformylation of an alpha-olefin unless otherwise stated. A “high” N/I ratio refers to an isomer ratio of at least 15/1; while a “low” N/I ratio refers to an isomer ratio of less than 15/1.
An N/I ratio is informative in describing a relative amount of the n-aldehyde in an isomeric mixture; however, differences in N/I ratios are not as informative in describing differences in percentages of the one or more iso-aldehydes in different isomeric mixtures. For example, an isomeric mixture with an N/I ratio of 50/1 contains only about 1% more iso-aldehydes than an isomeric mixture with an N/I ratio of 100/1; while an isomeric mixture with an N/I ratio of 2/1 contains about 31% more iso-aldehydes than an isomeric mixture with an N/I ratio of 50/1. Therefore, both N/I ratios and percentages of individual aldehydes are used to describe aldehyde isomeric mixtures.
The organophosphorus ligand provides predominant control over the N/I ratio achievable by its corresponding rhodium complex catalyst. Rhodium complex catalysts of organomonophosphorus ligands generally produce N/I ratios below 15/1. For example, rhodium-triphenylphosphine ligand complex catalysts are known to produce a limited N/I ratio from about 6/1 to about 12/1 from an alpha-olefin, depending primarily on the molar ratio employed of triphenylphosphine to rhodium, with high N/I ratios achieved only with high molar ratios of triphenylphosphine to rhodium, for example, greater than 200/1. Rhodium-tri(2,4-di-tert-butylphenyl)phosphite complex catalyst produces even a narrower N/I ratio range of generally from 0.5/1 to 3/1 depending on the reaction conditions.
Rhodium complex catalysts of certain organopolyphosphite ligands produce N/I ratios above 15/1 in hydroformylation processes, and consequently, these catalysts are subjects of both industrial interests and academic studies. U.S. Pat. Nos. 4,769,498 and 4,748,261 exemplify such industrial interests, while a publication by Annemiek van Rooy et al. in Organometallics (1996, 15, 835-847) exemplifies such academic studies. The latter discloses 1-octene hydroformylation results using rhodium complex catalysts of bulky diphosphites (also known as bisphosphites), including ligands A, B and C represented by the following formulas:

The 1-octene hydroformylation results in the Organometallics publication include selectivity data towards normal and branched aldehydes using the rhodium complex catalysts of ligands A, B and C under specified reaction conditions (see Table 1 of the publication). The selectivity data translate to N/I ratios of greater than 51/1 from ligands A and B, and 19/1 from ligand C. The publication also reports that the selectivity for the normal aldehyde decreases with increasing carbon monoxide partial pressure when using the rhodium complex catalyst of ligand C (see Table 3 of the publication).
Ligands A and B, as free ligands, are symmetric with the two phosphites in each ligand being equivalent both electronically and sterically. Ligand C, in contrast, is unsymmetrical with one phosphite being less bulky than the other. Upon mixing with a source of transition metal, such as Rh(CO)2acetylacetonate, the symmetric diphosphites give rise to chelating coordination immediately or after stirring for a few minutes, while the unsymmetrical diphosphite ligand C coordinates to the rhodium with its less bulky phosphite first and only forms chelating coordination after heating or evacuating to remove CO, as described in the Organometallics publication. The chelating coordination property of the symmetric diphosphites is preferred, because it enables a catalyst preparation by simply mixing the diphosphite and the source of transition metal. Symmetrical diphosphite ligands are preferred in commercial operations; see, for example, US Patent Publication 20060058558A1.
Substituted or unsubstituted biphenols are common linking groups for synthesizing diphosphites; see, for example ligands A, B and C hereinabove, and diphosphite ligands in U.S. Pat. Nos. 4,769,498 and 4,748,261. Derivatized calixarenes are also useful linking groups for synthesizing diphosphites to be used in hydroformylation processes. See, for example, U.S. Pat. No. 5,717,126 and unpublished International Application PCT/US2008/59216, filed on Apr. 3, 2008 in the name of Dow Global Technologies Inc. for “A CALIXARENE BISPHOSPHITE LIGAND FOR USE 1N HYDROFORMYLATION PROCESSES,” as well as publications by S. Steyer, et al., Dalton Transactions, 2005, 1301-1309; and C. Kunze, et al., Z. Anorg. Allg. Chem., 2002, 628, 779-787.
By far the most important oxo chemical is n-butyraldehyde with world wide annual consumption of more than 50% of all oxo aldehydes by weight, based on the total weight of all oxo aldehydes consumed. Although world wide annual consumption of iso-butyraldehyde is only about 15% of that of n-butyraldehyde, at certain times in a chemical market cycle, it may be desirable to increase production of iso-butyraldehyde and decrease production of n-butyraldehyde of an oxo aldehyde plant. It is therefore desirable to have a hydroformylation process that produces two or more aldehydes with a selectable N/I ratio by controlling processing parameters.
To satisfy the ever changing market demands for isobutyraldehyde and n-butyraldehyde, the art teaches using a mixture of two ligands—a high N/I ligand, for example, a diphosphite, and a low N/I ligand, for example, a monophosphite, to carry out the hydroformylation process. See, for example, US Patent Application Publication 20070123735A1 and unpublished International Application PCT/US2008/056602, filed on Mar. 12, 2008 in the name of Union Carbide Chemicals & Plastics Technology LLC for “HYDROFORMYLATION PROCESS WITH IMPROVED CONTROL OVER PRODUCT ISOMERS.”